Polyamide resins have been developed typically as additives and curing agents which modify properties of epoxy resin and the like, and epoxy compositions containing it as a component become generally cured products excellent in heat resistance, mechanical properties, chemical resistance and the like, and are utilized in the wide range of fields of adhesive, coating, laminated sheet, molding material, casting material and the like. Conventionally, the epoxy resin having been used most commonly as a main component of such epoxy resin compositions includes bisphenol A type epoxy resin. Further, acid anhydrides and amine compounds are known as curing agents for epoxy resin, while a phenolic novolak excellent in electric reliability is often used in the fields of electrical and electronic parts in view of heat resistance and the like. In addition, a nitrile rubber-based additive is used for the purpose of imparting flexibility to cured products, and above all, a nitrile rubber-modified polyamide is known as a heat resistant and flexibility additive.
However, cured products of a composition containing a nitrile rubber-based additive have excellent flexibility, while they result in decreasing water fastness and electric reliability. In addition, cured products of an epoxy resin where a phenolic novolak is used as a curing agent have excellent reliability, while the cured products are rigid and lack flexibility. For forms of recent electrical and electronic parts, not only conventional large packages and plates using a rigid board where the substrate is a glass fiber but also polyimide films, PET (polyethylene terephthalate) films or moldings of a flexible sheet resin substrate have been developed, and these are utilized for flexible printed wiring boards as a laminated structure contacting a metal foil or a metallic circuit. Said laminated body is produced typically by coating an epoxy resin composition in the state of varnish onto a substrate of polyimide, copper foil or the like, removing the solvent and then curing the coated epoxy resin composition. Sufficient flexibility and high adhesiveness to a substrate of polyimide, copper foil or the like, as well as heat resistance and electric reliability are being required for the resins (resin compositions and cured products) used in this case.
It is known that when a phenolic hydroxy group-containing rubber-modified polyamide resin is added in a composition as a flexibility additive for the purpose of improving fragility of conventional epoxy resin compositions, the cured products have such properties as heat resistance and toughness (Patent Literature 1 and Patent Literature 2). In addition, Patent Literatures 3 and 4 describe that epoxy resin compositions comprising an epoxy resin and a phenolic hydroxy group-containing polyamide resin have excellent fire retardance and are also useful as materials for flexible printed wiring boards.
However, as regards the phenolic hydroxy group-containing polyamide resin disclosed in these literatures, the condensation of a diamine component and a dicarboxylic acid component is carried out in the presence of a phosphorus acid compound and a phosphorus ion and water are apt to remain due to the nature of the resin described in said Patent Literatures, resulting in that the residual amount of phosphorus ion and the remaining amount of water are high even after washing the resin. In addition, in the case of said high molecular weight polyamide resin, it is more difficult to wash and dry it sufficiently due to its increased viscosity as the molecular weight is higher, resulting in that a phosphorus ion and water are further apt to remain. Such remaining water and phosphorus ion cause deterioration of electrical properties when said resin is used for electrical and electronic parts.    [Patent Literature 1] JP H02-245032    [Patent Literature 2] JP H3-47836    [Patent Literature 3] WO 2004/048436    [Patent Literature 4] WO 2002/00791